The publication ‘Control of Thyristor-Based Commutation Cells’ published in the context of the IEEE Energy Conversion Congress and Exposition (Raleigh, N.C., USA, September 2012) describes a method for control of a commutation cell. The described system arrangement comprises a transformer with a primary side consisting of a main winding and a regulating winding and a secondary side with a secondary winding. The regulating winding of the primary side can be connected with the main winding by means of an arrangement consisting of four semiconductor switching elements in three different forms. In a first switching mode the regulating winding can be connected in series with the main winding. In a second switching mode the regulating winding can be connected with the main winding in opposition. In a third switching mode the regulating winding can be disconnected from the circuit, i.e. no current is conducted by the regulating winding. Accordingly, the four semiconductor switching elements and the regulating winding together form a switching module. The switching module comprises two commutation cells. Each commutation cell comprises an upper and a lower current path. The two current paths can each be switched to the state ‘conducting’ or ‘blocking’ by a thyristor pair in antiparallel connection. Depending on the state in which the thyristor pair is disposed, either the upper or the lower path constantly conducts current.
The publication describes two possible commutation strategies for commutation of a current from an initially conducting path to an initially blocking path. According to a first commutation strategy, as soon as conditions favourable to commutation prevail a previously defined commutation instant is waited out. When this commutation instant is reached, the thyristor pair previously switched to be conducting is deactivated and a dead time is waited out, which ensures that no unexpected back-triggering of the thyristors occurs. After expiry of the dead time, the previously blocking thyristor pair is activated, commutation of the current from the previously conducting path to the previously blocking path takes place and the commutation is concluded. If favourable conditions for commutation are not fulfilled, commutation in accordance with a second commutation strategy, which is also termed forced commutation or commutation by way of short-circuit, is initiated.
U.S. Pat. No. 3,619,765 describes a regulating unit and a method for connecting winding taps of a transformer with an electrical mains. The transformer connects an alternating voltage source with a load. A semiconductor switching element is associated with each winding tap of the transformer and respectively consists of two thyristors in antiparallel arrangement.
According to the method, switching actions increasing the load voltage are allowed only when the flow of power is directed from the alternating voltage source to the load. Switching actions reducing the load voltage are allowed only when the flow of power is from the load to the alternating voltage source. In addition, switching actions in the vicinity of the zero transitions of load current and alternating voltage are supressed. In this way, short-circuits of the winding taps during switching over from a first winding tap to a second winding tap are precluded.
The state of the art methods for control of commutation cells in that regard do not take into consideration distorted, particularly rapidly changing, load currents. Distorted load currents in the case of the state of the art methods can have the consequence of unexpected short-circuits of the winding parts, which are to be connected, due to unexpected change of current direction.
Thyristors are power semiconductors which in electronic power systems are designed for switching high electrical currents and voltages. The following terms are used hereinafter in connected with a thyristor:
‘main voltage’ is the voltage between an anode and a cathode of a thyristor;
‘main current’ is the current through the cathode of a thyristor;
‘control voltage’ is the voltage between the gate and the cathode of a thyristor, this being positive when the gate has the higher potential by comparison with the cathode;
‘control current’ is the current through the gate of a thyristor;
‘trigger current’ is a control current flowing into the gate, the control current of a thyristor then being positive;
‘forward direction’ is the direction from anode to cathode;
‘forward voltage’ is the main voltage poled in forward direction, the main voltage then being positive;
‘forward current’ is the main current flowing in forward direction, the main current then being positive;
‘reverse direction’ is the direction from cathode to anode;
‘reverse voltage’ is the main voltage poled in reverse direction, the main voltage then being negative;
‘blocking state’ and ‘conducting state’ are the two stable operating states which the thyristor can adopt when a forward current flows; and
‘latching current’, ‘holding current’ and ‘recovery time’ are characteristic variables of the respective thyristor and are usually indicated in the data sheet thereof.
A thyristor can be brought into the conducting state, which is also termed triggering or switching on, in that a forward voltage is applied and a positive control voltage is applied at least temporarily, typically for approximately 10 μs, and a trigger current generated until the forward current exceeds the latching current. The thyristor now remains in the conducting state even when the control current extinguishes or even is reversed in polarity, but only as long as the forward current does not exceed the holding current and the forward voltage is applied. However, as soon as the main voltage is reversed in polarity so that there is a reverse voltage from the forward voltage or the forward current drops below the holding current the thyristor goes over into the blocking state, which is also called extinguishing or switching off. In addition, in the following ‘deactivate’ means deactivation of the gate drive so as to prevent triggering of the thyristor. Correspondingly, ‘activate’ means to make triggering possible by driving of the gate of the thyristor.
Normal thyristors, which are also termed naturally commutating thyristors, can be extinguished only in the afore-described manner. Gate turn-off (GTO) thyristors can additionally also be extinguished in that an extinguishing current is generated. Thus, normal thyristors usually extinguish at the earliest when the main current reaches the next zero transition, whereagainst GTO thyristors can be extinguished at any desired instant. Normal thyristors are at present designed for, for example, currents up to 2.2 kA and more and for voltages up to 7 kV and more, whereas GTO thyristors at present can be designed only for lower currents and voltages and are more expensive.
After extinguishing, the thyristor needs a certain period of time until it can again accept a voltage in forward direction. This time period is termed hold-off interval and is formed from the sum of the recovery time and a safety margin.
As control of a commutation cell it is to be generally understood in the following that the commutation cell is driven in such a way that the desired switching state can be reliably achieved.
Moreover, a distinction can be made between normal commutation and forced commutation. The term “normal commutation” is used when the current through the previously conducting switching element of a commutation cell commutates directly and within a short time to the previously blocking switching element. In that regard, the physical commutation, in which the current commutates from one load branch to the other, takes place—in normal commutation—in a time range with respect to which load current and induced voltage have different signs. By contrast, in the case of forced commutation the commutation takes place by way of an intentional transient short-circuit. During the short-circuit current which builds up and diminishes again after the voltage zero transition, the previously conducting switching element or thyristors thereof has or have sufficient time to recover. By contrast to normal commutation, forced commutation can be carried out at any time.